EMI shield that adheres to and conforms with printed circuit board surfaces

ABSTRACT

An electrically continuous, grounded conformal EMI protective shield and methods for applying same directly to the surfaces of a printed circuit board. The EMI shield adheres and conforms to the surface of the components and printed wiring board. The shield takes the shape of the covered surfaces while adding little to the dimensions of the surfaces. The EMI shield includes low viscosity, high adherence conductive and dielectric coatings each of which can be applied in one or more layers using conventional spray techniques. The conductive coating prevents substantially all electromagnetic emissions generated by the shielded components from emanating beyond the conformal coating. The dielectric coating is initially applied to selected locations of the printed circuit board so as to be interposed between the conductive coating and the printed circuit board, preventing the conductive coating from electrically contacting selected components and printed wiring board regions. A high viscosity, non-electrically-conductive filler material is applied to printed circuit board regions that have surfaces that are cavitatious and/or which have a highly variable slope. The filler material can be used in conjunction with conformal EMI shield board level coating. The high viscosity, electrically non-conductive filler material substantially covers each cavity such that the covered cavity is inaccessible and that the covered region of the printed circuit board has a contiguous, contoured surface. A pre-manufactured non-electrically-conductive component cover can be mounted over a corresponding component and secured to the printed wiring board. The component cover and printed wiring board surround the component, forming a sealed enclosure. The component cover has a thin cross-section and an interior surface that follows closely the surface of the component. This minimizes the volume enclosed by the component cover. In addition, the exterior surface of the component cover has a low profile, and prevents the conformal EMI shield from physically contacting the covered component. Instead, the exterior surface of the component cover is coated with the EMI shield. This enables the covered component to be removed from the printed circuit board for repair, replacement or salvage without having to risk damage to the printed wiring board or component that may occur with the removal of a conformal EMI shield applied directly to the component.

RELATED APPLICATIONS

[0001] The present application is related to the following commonlyowned U.S. Patent Applications:

[0002] U.S. Patent Application entitled “FILLER MATERIAL ANDPRETREATMENT OF PRINTED CIRCUIT BOARD COMPONENTS TO FACILITATEAPPLICATION OF A CONFORMAL EMI SHIELD,” naming as inventor Lowell E.Kolb and filed concurrently herewith under Attorney Docket No.10001844-1; and

[0003] U.S. Patent Application entitled “A LOW PROFILENON-ELECTRICALLY-CONDUCTIVE COMPONENT COVER FOR ENCASING CIRCUIT BOARDCOMPONENTS TO PREVENT DIRECT CONTACT OF A CONFORMAL EMI SHIELD,” namingas inventor Lowell E. Kolb and filed concurrently herewith underAttorney Docket No. 10010907-1.

BACKGROUND OF THE INVENTION

[0004] 1. Field of the Invention

[0005] The present invention relates generally to electromagneticinterference (EMI) protective measures and, more particularly, EMIprotective measures for printed circuit boards.

[0006] 2. Related Art

[0007] Most countries in the world have regulations that limit theamount of electromagnetic emissions that electromagnetic equipment mayproduce. Electromagnetic emissions are the unwanted byproduct ofhigh-frequency electronic signals necessary, for example, to operate anelectronic microprocessor or other logic circuitry. The electromagneticinterference (EMI) that results is a problem when it interferes withlicensed communications, such as television, radio, air communicationsand navigation, safety and emergency radios, etc. This type ofinterference has also been known as radio-frequency interference (RFI).See CFR 47 part 15 and ANSI publication C63.4-1992 for regulations inthe United States, or CISPR publication 11 or 22 for internationalregulations. Also, “Noise Reduction Techniques in Electronic Systems” byHenry W. Ott, serves as an excellent reference on the current art forthe control of EMI, and the broader topic known as electromagneticcompatibility (EMC).

[0008] To meet EMI regulations, most electronic equipment currentlyemploys a combination of two approaches commonly referred to as ‘sourcesuppression” and “containment.” Source suppression attempts to designcomponents and subsystems such that only essential signals are presentin signal interconnections, and that all non-essential radio frequency(RF) energy is either not generated or attenuated before it leaves thecomponent subsystem. Containment attempts to place a barrier around theassembled components, subsystems, interconnections, etc., so that anyunwanted electromagnetic energy remains within the boundaries of theproduct, where it is dissipated harmlessly.

[0009] This latter approach, containment, is based on a principle firstidentified by Michael Faraday (1791-1867), that a perfectly conductingbox that completely encloses a source of electromagnetic emissions wouldprevent those emissions from leaving its boundaries. This principle isemployed in conventional shielded cables as well as in shieldedenclosures. Conventional shielded enclosures usually consist of a metalbox or cabinet that encloses the equipment. The metal box is oftensupplemented with additional features as necessary in an attempt to keepRF energy from escaping via the power cord and other interconnectingcables. The metal shield may be structural, for example, the productenclosure itself. For example, a product enclosure might consist of aplastic structure with a conductive coating on the surface. Thisapproach is commonly implemented in, for example, cell phones. Morecommonly, the metal shield is implemented as a metal “cage” inside theproduct enclosure since the EMI suppression required for the entireproduct or system requires that only a portion of the product beshielded. Such metallic cages are placed around components, or aroundsubsystems when additional EMI reduction is required.

[0010] There are numerous drawbacks to the use of such metallic boxes.These drawbacks are primarily related to the lack of shieldingeffectiveness provided by conventional metallic boxes. For example, themetallic box creates a stagnant buffer of insulating air around thecomponent causing the temperature of the component to increase. In suchproducts, the enclosure typically includes cooling holes and fans tocirculate air around the metallic box to dissipate the heat. Inaddition, electromagnetic energy often escapes the shield at gapsbetween the shield and the printed circuit board. Electrical gaskets andspring clips have been developed to minimize such leakage.Unfortunately, they increase the cost and complexity of the printedcircuit board, and have limited success. In addition, leakage occursbecause the cables and wires penetrating the shield are not properlybonded or filtered as they exit the metallic box. Further drawbacks ofmetallic cages include the added cost and weight to the printed circuitboard assembly, as well as the limitations such metallic boxes place onthe package design.

SUMMARY OF THE INVENTION

[0011] The present invention is directed to an electrically continuous,grounded conformal electromagnetic interference (EMI) protective shield,methods for applying same directly to the surfaces of a printed circuitboard, and a printed circuit board designed to be coated with such aconformal EMI shield. The EMI shield of the present invention adheres toand conforms with the surface of the components and printed wiring boardto which it is applied. Because the conformal EMI shield is relativelythin, the conformal EMI shield takes the shape of the covered componentswithout changing significantly the dimensions of the printed circuitboard regions to which it is applied. The EMI shield of the presentinvention includes two primary coatings. A conductive coating preventssubstantially all electromagnetic radiation from passing through theconductive coating, whether generated by the shielded components oremanating from a source not on the printed circuit board. The conformalEMI shield also includes a dielectric coating interposed between theconductive coating and the printed circuit board to prevent theconductive coating from electrically contacting predetermined portionsof the coated printed circuit board region.

[0012] Advantageously, the conformal EMI shield of the present inventionprovides significantly improved shielding effectiveness as compared withconventional techniques of placing localized shielding boxes overcritical components or subassemblies. In contrast to such approaches,the conformal EMI shield does not suffer from “leaks” where the shieldattaches to the board because he shield coats the printed circuit boardcompletely; that is, there are no gaps, voids or breaks of any size inthe shield.

[0013] Another advantage of the present invention is that it does notcreate a thermal insulation of “dead air” space around the shieldedcomponents. In fact, because the conformal EMI shield is a thin,continuous layer that is physically attached to the printed circuitboard, it actually promotes the distribution of heat away from thecoated printed circuit board regions rather than serving as a thermalinsulator. Specifically, the conformal EMI shield conducts heat awayfrom the component to the surface of the conductive coating where it iseither dissipated through convection to the surrounding environment orconducted to a heat sink.

[0014] As noted, conventional product enclosures include cooling holesand fans to circulate air around the printed circuit board and metallicEMI boxes. An associated benefit of the present invention is that thesize restrictions on the cooling holes and fan grills on the productenclosures is eliminated since there is no longer a need to remove heatfrom a high temperature metallic EMI box on the printed circuit board.

[0015] A further advantage of the present invention is that iteliminates the need for all other types of EMI shielding components. Inparticular, by eliminating the conventional metallic EMI boxes reducesthe cost and the weight of the sheet metal. This, in turn, eliminatesthe constraints on package design imposed by such conventionalapproaches. Furthermore, the associated shielding components such asgaskets and spring contacts are eliminated.

[0016] A number of aspects of the invention are summarized below, alongwith different embodiments that may be implemented for each of thesummarized aspects. It should be understood that the embodiments are notnecessarily inclusive or exclusive of each other and may be combined inany manner that is non-conflicting and otherwise possible. It shouldalso be understood that these summarized aspects of the invention areexemplary only and are considered to be non-limiting. Also, variousaspects of the present invention and embodiments thereof provide certainadvantages and overcome certain drawbacks of conventional techniques.Not all aspects and embodiments share the same advantages and those thatdo may not share them under all circumstances. These disclosed aspects,some of which are summarized below, are not to be construed as limitingin any regard; they are provided by way of example only and in no wayrestrict the scope of the invention.

[0017] In one aspect of the invention, an electrically continuousconformal EMI protective shield for coating a region of a printedcircuit board is disclosed. The conformal EMI shield includes a lowviscosity, high adherence dielectric coating configured to be applieddirectly to surfaces of one or more regions of the printed circuitboard. The dielectric coating configured to provide a layer ofinsulation that adheringly coats all surfaces of the printed circuitboard region. A low viscosity conductive coating configured to beapplied at least to the dielectric coating to prevent electromagneticemissions generated by the printed circuit board from emanating beyondthe conformal coating. The EMI shield adheres directly to and conformswith the surface of the printed circuit board region.

[0018] The dielectric coating has a combination of adhesion andviscosity that, when applied, enables the dielectric coating to adhereto all exposed surfaces of a printed circuit board to which it isapplied. Preferably, the dielectric coating is thixotropic. In onespecific embodiment, the dielectric coating has a combination ofviscosity and adhesion properties sufficient to enable the dielectriccoating to be applied via atomization spray techniques and to adhere tothe surface in the immediate vicinity of where it was applied. Thedielectric coating can be formed with a plurality ofsuccessively-applied layers of dielectric material. In sum, dielectriccoating covers completely all surfaces including those that definecavities as well as those that have highly variable surface tangentssuch as very sharp edges.

[0019] Preferably, the dielectric coating and the conductive coatinghave similar composite resin structures. In one preferred embodiment,the dielectric coating is Clear Water Reducible Barrier Coat, FormulaNumber CQW-L200DF, and the conductive coating is TARA EMI-RFI shielding,Formula MQW-L85, both manufactured by The Egyptian Lacquer ManufacturingCompany, Franklin, Tenn., USA.

[0020] In another aspect of the invention, an electrically continuousconformal coating for shielding a plurality of regions of a printedcircuit board from electromagnetic interference is disclosed. Thecontinuous conformal coating includes a conductive coating having anohmic resistance sufficient to prevent electromagnetic waves frompassing therethrough. The conductive coating adheringly and conforminglycoats the surface of each printed circuit board region. A dielectriccoating is interposed between the conductive coating and predeterminedportions of each printed circuit board region. The dielectric coatingcompletely insulates the predetermined portions of the printed circuitboard. The conductive coatings of each printed circuit board region areelectrically connected to conductive coatings conforming coating andsecured to the surfaces of other regions of the printed circuit board.

[0021] In another aspect of the invention a printed circuit board isdisclosed. The printed circuit board includes a printed wiring board, aplurality of components mounted on the printed wiring board and anelectrically continuous conformal coating for providing anEMI-impervious shield conformingly and adheringly on the printed circuitboard. The shield includes a conductive coating that prevents theelectromagnetic waves from passing therethrough. The conductive coatingconformingly and adheringly coats the surface of one or more regions ofthe printed circuit board. The conductive coating of each region iselectrically connected to each other. A dielectric coating is interposedbetween the conductive coating and predetermined portions of eachprinted circuit board region. The dielectric coating completelyinsulates the predetermined portions of said printed circuit boardregion.

[0022] In a still further aspect of the invention a method for coating aprinted circuit board is disclosed. The method includes providing aprinted circuit board; and coating the printed circuit board with anelectrically continuous conformal coating for providing anEMI-impervious shield conformingly and adheringly on the printed circuitboard. The shield includes a conductive coating that prevents theelectromagnetic waves from passing therethrough. The conductive coatingconformingly and adheringly coats the surface of one or more regions ofthe printed circuit board. The conductive coating of each region iselectrically connected to each other. A dielectric coating is interposedbetween the conductive coating and predetermined portions of eachprinted circuit board region. The dielectric coating completelyinsulates the predetermined portions of said printed circuit boardregion.

[0023] Further features and advantages of the present invention as wellas the structure and operation of various embodiments of the presentinvention are described in detail below with reference to theaccompanying drawings. In the drawings, like reference numerals indicateidentical or functionally similar elements. Additionally, the left mostone or two digits of a reference numeral identify the drawing in whichthe reference numeral first appears.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The foregoing and other features and advantages of the presentinvention will be understood more clearly from the following detaileddescription and from the accompanying figures. This description is givenby way of example only and in no way restricts the scope of theinvention. In the figures:

[0025]FIG. 1 is a cross-sectional view of one aspect of the conformalEMI shield of the present invention illustrating its conductive anddielectric coatings.

[0026]FIG. 2A is a side cross-sectional view of an integrated circuitmounted on a printed wiring board and covered with a conformal EMIshield in accordance with one embodiment of the present invention.

[0027]FIG. 2B is a top cross-sectional view of the integrated circuitintroduced in FIG. 2A taken along section line I-I, showing only thedielectric coating portion of the conformal EMI shield of the presentinvention applied to the integrated circuit.

[0028]FIG. 2C is a top cross-sectional view of the integrated circuitillustrated in FIG. 2A taken along section line I-I, showing theconductive coating portion of the conformal EMI shield of the presentinvention applied over the dielectric layer shown in FIG. 2B.

[0029]FIG. 3 is a side cross-sectional view of a printed wiring boardwith various components mounted thereon with one embodiment of theconformal EMI shield illustrated in FIG. 1 applied thereto.

[0030]FIG. 4 is a cross-sectional view of a shielded connector such asthat shown in FIG. 3 with a ground moat mounted on the printed wiringboard that surrounds the connector and is covered by the conformal EMIshield of the present invention.

[0031]FIG. 5 is a cross-sectional view of a ground pad mounted on theprinted wiring board and covered by the conformal EMI shield of thepresent invention.

[0032]FIG. 6A is a cross-sectional view of an edge region of a printedwiring board showing a continuous conformal EMI shield of the presentinvention coating the top, edge and bottom surfaces of the printedwiring board.

[0033]FIG. 6B is a cross-sectional view of an edge region of a printedwiring board showing the conformal EMI shield coating ground stripsmounted on the top and bottom surface proximate to the edge surfaces onwhich a grounded edge plating is mounted.

[0034]FIG. 6C is a cross-sectional view of an edge region of a printedwiring board showing the conformal EMI shield coating ground stripsmounted on the top and bottom surface proximate to the edge surfaceswith the ground strips connected to a ground plane through ground vias.

[0035]FIG. 6D is a cross-sectional view of an edge region of a printedwiring board showing the conformal EMI shield coating ground stripsmounted on the top and bottom surface proximate to the edge surfaceswith a spring clip electrically connecting the two ground moats.

[0036]FIG. 6E is a cross-sectional view of an edge region of a printedwiring board showing the conformal EMI shield coating the top and bottomsurfaces with a spring clip electrically connecting the two conformalEMI shield regions.

[0037]FIG. 7 is a custom memory card coated with the conformal EMIshield in accordance with one embodiment of the present invention.

[0038]FIG. 8A is a cross-sectional view of a printed wiring board with acomponent mounted thereon with a nonconductive component cover mountedover the component to encase the component in a compartment defined bythe cover and the printed wiring board.

[0039]FIG. 8B is a cross-sectional view of a printed wiring board with aprocessor mounted thereon with a nonconductive, conformal cover with acontoured, arbitrary shape mounted over the processor to encase theprocessor in a compartment defined by the cover and the printed wiringboard.

[0040]FIG. 8C is a cross-sectional view of the printed wiring board andcomponent compartment shown in FIG. 8B with a dielectric coating of thepresent invention covering the surface of the component cover andsurrounding printed wiring board.

[0041]FIG. 8D is a cross-sectional view of the printed wiring board andcomponent compartment shown in FIG. 8C with a conductive coating of thepresent invention covering the dielectric coating, forming conformal EMIshield of the present invention.

[0042] FIGS. 8E-1 and 8E-2 (collectively, FIG. 8E) are cross-sectionalviews of two embodiments of the component cover shown in FIG. 8Aillustrating different embodiments of a line or severability in the formof a crease.

[0043]FIG. 9A is a cross-sectional view of a printed circuit board witha filler material applied to certain regions thereof in accordance withone embodiment of the invention to cover, encapsulate enclose orotherwise coat cavities on the printed circuit board, such as betweenthe components and printed wiring board.

[0044]FIG. 9B is a top perspective view of a void formed in the fillermaterial shown in FIG. 9A.

[0045]FIG. 9C is a cross-sectional view of a printed circuit board withthe filler material applied thereto, as shown in FIG. 9A, with thedielectric coating of the present invention applied to the surface ofthe filler material and neighboring printed wiring board surfaces.

[0046]FIG. 9D is a cross-sectional view of the printed circuit boardwith a filler material and the dielectric coating applied thereto, asshown in FIG. 9C, with the conductive coating of the present inventionapplied to the surface of the dielectric coating to form the conformalEMI shield of the present invention.

[0047]FIG. 10 is a flow chart of the operations performed to manufacturean EMI-shielded printed circuit board in which component covers andfiller material are utilized with the conformal EMI shield in accordancewith one embodiment of the present invention.

[0048]FIG. 11 is a flow chart of the primary operations performed inutilizing a component cover shown in FIGS. 8A-8E with the conformal EMIshield introduced in FIG. 1.

DETAILED DESCRIPTION Table of Contents

[0049] 1. Introduction

[0050] 2. Conformal EMI Shield Materials

[0051] A. Overview

[0052] B. Dielectric Coating

[0053] C. Conductive Coating

[0054] 3. A Printed Circuit Board With A Conformal EMI Shield

[0055] A. General

[0056] B. Printed Circuit Board Coverage

[0057] C. Grounding of Conformal EMI Shield

[0058] D. Electrically Connecting EMI Shielded Regions

[0059] E. Design of Printed Wiring Board to Accommodate EMI Shield

[0060] 4. Individual Components Coated with Conformal EMI Shield

[0061] 5. A Low Profile Component Cover For Encasing Components

[0062] 6. Filler Material For Use With Board-Level Containment ofElectromagnetic Emissions

[0063] 7. Manufacturing of Printed Circuit Board With Conformal EMIShield

[0064] 8. Closing

[0065] 1. Introduction

[0066] The present invention is directed to an electrically continuousconformal EMI protective shield and methods for applying same directlyto the surfaces of one or more regions of a printed circuit board. Whenthe EMI shield of the present invention is applied to the surface of aprinted circuit board, it adheres to and conforms with the surface ofthe components and printed wiring board to which it is applied. Theconformal EMI shield conformally coats the printed circuit boardsurfaces while not changing substantially the dimensions of the coatedprinted circuit board regions. The EMI shield includes low viscosity,high adherence conductive and dielectric coatings each of which can beapplied in one or more layers using conventional spray atomizationtechniques. The shield's conductive coating prevents substantially allelectromagnetic emissions generated by the shielded components fromemanating beyond the conformal coating. It also prevents electromagneticemissions generated externally from interfering with the coated printedcircuit board regions. The dielectric coating is initially applied toselected locations of the printed circuit board so as to be interposedbetween the conductive coating and the printed circuit board, preventingthe conductive coating from electrically contacting selected portions ofthe printed circuit board regions.

[0067] Aspects of the conformal EMI shield also include a highviscosity, non-electrically-conductive filler material for applicationto printed circuit board regions that have surfaces that are cavitatiousand/or which have sharp edges or other highly variable surface tangentslopes. The filler material and associated methodologies of the presentinvention are preferably used in conjunction with the noted conformalEMI shield. The high viscosity, electrically non-conductive fillermaterial substantially covers, and preferably infills, each cavity suchthat the covered cavity is thereafter inaccessible. The filler materialalso coats the sharp edges on the printed circuit board. Thus, thepretreated portions of the printed circuit board regions have acontiguous, contoured surface that facilitates the coating of theprinted circuit board regions with the conformal EMI shield.

[0068] In certain aspects of the invention, the invention includes apre-manufactured non-electrically conductive component cover. Thiscomponent cover is configured for placement over a printed circuit boardcomponent and secured to the printed wiring board. The component coverand printed wiring board surround the component, forming an enclosurereferred to as a component compartment. The component cover has asubstantially thin cross-section and an interior surface that followsclosely the surface of the component, thereby minimizing the volumeenclosed by the component cover. In addition, the interior surface ofthe component cover is immediately adjacent to the component so as notto add significantly to the dimensions of the printed circuit board. Assuch, the component cover has a low profile and prevents thesubsequently applied conformal EMI shield from physically contacting theencased component. Instead, the exterior surface of the component coveris coated with the EMI shield. This provides the significant benefits ofthe conformal EMI shield while providing access to the compartmentalizedcomponent. This enables the covered component to be removed from theprinted circuit board for repair, replacement or salvage without havingto risk damage to the printed wiring board or component that may occurwith the removal of a conformal EMI shield applied directly to thecomponent.

[0069] 2. Conformal EMI Shield Materials

[0070] A. Overview

[0071] As noted, the conformal EMI shield includes a conductive coatingand a dielectric coating permanently bonded to each other. The materialsthat can be used in the conductive and dielectric coatings are describedbelow with reference to FIGS. 1-3. FIG. 1 is a cross-sectional view ofone embodiment of the conformal EMI shield of the present invention.FIG. 2A is a cross-sectional view of an integrated circuit componentmounted on a printed wiring board forming a portion of a printed circuitboard. The integrated circuit component and printed wiring board havebeen coated with one embodiment of the conformal EMI shield of thepresent invention. FIG. 2B is a top view of the integrated circuitcomponent illustrated in FIG. 2A taken along section line I-Iillustrating the application of the shield's dielectric coating inaccordance with one embodiment of the present invention. FIG. 2C is atop view of the integrated circuit component taken along the samesection line illustrating the application of the shield's conductivecoating in accordance with one embodiment of the present invention.

[0072] Referring now to FIG. 1, this embodiment of EMI shield 100includes a dielectric coating 102 and a conductive coating 104. Theexposed surfaces of selected printed circuit board regions are coatedwith conformal EMI shield 100. Such surfaces can be, for example, thetop, side and, if exposed, bottom surface of a component, the surface ofany leads, wires, etc, that are connected to the component, as well asany other exposed surface of any other portions, elements, sections orfeatures (hereinafter “features”) of the components and printed wiringboard located in the coated printed circuit board region. It should beappreciated that the identification and selection of the propercombination of material properties for both, dielectric coating 102 andconductive coating 104 is important to achieving a conformal EMI shieldthat can be applied directly to the surface of the printed circuit boardwithout damaging the printed circuit board components and connections,that does not expose the coated regions to risk of electrical shorts andthat completely envelops or encases the coated regions to provide adesired shielding effectiveness. As will be described in detail below,conformal EMI shield 100 not only achieves such operational objectives,but does so, as noted, by directly coating; that is, physically adheringto, the surface of coated printed circuit board regions. This enablesconformal EMI shield 100 to conformingly coat the surfaces of theshielded printed circuit board regions.

[0073] B. Dielectric Coating

[0074] Dielectric coating 102 is comprised of a material that iselectrically nonconductive and, preferably, thermally conductive.Importantly, the material properties of dielectric coating 102,described in detail below, enable dielectric coating 102 to completelycoat and securely attach to the component and board surfaces to which itis applied. Generally, dielectric coating 102 is thixotropic.Specifically, the material properties of dielectric coating 102 includeprimarily a combination of viscosity and adhesion sufficient to enabledielectric coating 102 to be applied via atomization spray techniquesand, once applied, to adhere to the surface in the immediate vicinity ofwhere it was applied. In other words, adhesiveness of dielectric coating102 is sufficient to prevent dielectric coating 102 from separating fromthe surface to which it is applied, a phenomenon commonly referred to asdewetting. Such a condition will otherwise result in a void indielectric coating 102, providing the potential of an electrical shortin the exposed portion of printed circuit board 304. Dielectric coating102 can comprise multiple, successively applied layers of dielectricmaterial. As such, dielectric material 102 preferably also includes theproperties necessary to enable it to adhere to or bond with previouslyapplied dielectric layers.

[0075] More specifically and in one embodiment, dielectric coating 102has a viscosity of at least 45″ #2 Zahn Cup (full body). In anotherembodiment, dielectric coating 102 has a viscosity in the range of50-100″ #2 Zahn Cup (full body). In one preferred embodiment, dielectriccoating 102 has a viscosity of 70-95″ #2 Zahn Cup (full body). Adielectric coating 102 having any of the above viscosity values can beapplied uniformly using a conventional spray atomization technique. Thisenables dielectric coating 102 to completely access and coat thesurfaces of the components and board that are located underneathcomponent leads, between components and wiring board surfaces and otherregions that are exposed yet difficult to access. Such features of theprinted circuit board are referred to generally herein as cavities. Ingeneral, dielectric coating 102 can adhere to the materials utilized inthe printed circuit board. Such materials include, but are not limitedto, FR-4 such as polymethylmethacrylates, bisphenol-A based epoxy andfiberglass, ceramics such as aluminum oxide and silicon dioxide,silicon, polyimide (silicon wafers), polyethylene (sockets),polyethylene terephthalate, polystyrene (sockets), polyphenylsulfone orPPS (chip sockets), polyvinyl chloride or PVC (wire coverings), siliconerubbers such as RTV (various surfaces), aluminum, gold, stainless steeland low carbon steel), tin, lead, and others. Dielectric coating 102 hasan adhesion suitable to provide a thixotropic material at the givenviscosity. Dielectric coating 102 preferably has an adhesion thatenables it to pass the ASTM D-3359-83 Method A Tape Test using a 1″ (25mm wide) semi-transparent pressure-sensitive tape with and adhesionstrength of 25-70 and, more preferably, 30-50 ounces per inch whentested in accordance with ASTM Test Method D-3330.

[0076] In one embodiment, dielectric coating 102 is comprised primarilyof Clear Water Reducible Barrier Coat, Formula Number CQW-L200DF,manufactured by The Egyptian Coating Lacquer Manufacturing Company,Franklin, Tenn., USA. CQW-L200DF has a viscosity in the range of 50-60″#2 Zahn Cup (full body) and an adhesion that enables it to pass the ASTMD-3359-83 Method A Tape Test using a 1″ (25 mm wide) semi-transparentpressure-sensitive tape with an adhesion strength of 40±2.5, ounces perinch when tested in accordance with ASTM Test Method D-3330. CQW-L200DFprovides excellent adhesion to materials commonly found on a printedcircuit board including, but not limited to the materials noted above.

[0077] In certain applications there may be surfaces on printed circuitboard 304 that are more difficult to adhere to despite dielectriccoating 102 having a combination of properties noted above. Inparticular, cavities and very sharp or pointed surfaces provide lessopportunity for a material to adhere to the defining surfaces. In suchapplications, it is preferred that a conservative approach is taken withregard to coverage since incomplete coverage of the printed circuitboard can lead to an electrical short circuit when conductive coating104 is applied. Accordingly, in such applications, dielectric coating102 can be applied in multiple applications, each resulting in a layerof dielectric material coating the covered region of the printed circuitboard. For example, when dielectric coating 102 is the Egyptian CoatingCQW-L200DF, it is preferred that dielectric coating 102 is applied intwo applications of approximately 1 mil each, for a total thickness ofapproximately 2 mils. Each layer is preferably applied with 4 or 5cross-coats, with a delay or pause between the first and secondapplications of approximately 1 to 2 minutes to allow the first layer toset up before the second layer is applied. In such embodiments, theinitial layer may have a void located at the apex of a sharp edge orwithin a cavity. Each subsequent cross-coat of dielectric coating 102adheres to the prior layer as well as the underlying printed circuitboard surface, reducing the size of the void. Ultimately, the void isfilled or eliminated with a subsequent cross-coat or layer of dielectricmaterial. As is well known in the relevant arts, cross-coats areimplemented to insure uniform application of dielectric coating 102 wheneach layer of dielectric coating 102 is applied manually. However, suchcross-coats are not necessary when dielectric coating 102 is appliedwith robotic or other automated equipment. CQW-L200DF is preferablyapplied at room temperature, between 60-100 degrees Fahrenheit, andpreferably between 60-80 degrees Fahrenheit. Although CQW-L200DF can becured at room temperature, to expedite manufacturing processes and toremove any water-based components from dielectric coating 102,dielectric coating 102 is preferably thermally cured at an elevatedtemperature below that which the underlying printed circuit board canwithstand. It should be apparent to those of ordinary skill in the artthat dielectric coating 102 need only be cured to the extent necessaryto apply conductive coating 104. As will be described below, bothdielectric coating 102 and conductive coating 104 are thermally curedafter conductive coating 104 is applied.

[0078] It should be understood that the thickness of dielectric coating102 can differ from that noted, depending on the application. Forexample, in an alternative embodiment, dielectric coating 102 is formedwith 2 to 4 cross-coats for each of 4 layers of dielectric material,resulting in a thickness of approximately 6 to 10 mils. Thus, dielectriccoating 102 has a combination of adhesion and viscosity that enables itto form a uniform, contiguous surface over the coated surfaces with novoids formed therein.

[0079] An example of dielectric coating 102 applied to the integratedcircuit shown in FIG. 2A is illustrated in FIG. 2B. As shown therein,dielectric coating 102 adheres to the entire exposed surface ofintegrated circuit leads 208, including those lead surfaces that areadjacent to and facing the side surface of integrated circuit 204. Inaddition, dielectric coating 102 coats the side surfaces of integratedcircuit body 204 that are accessible only through gaps betweenneighboring leads 208. Note that the thickness of dielectric coating 102may vary slightly, being greater where access is more direct.Nevertheless, dielectric coating 102 completely coats the entire exposedsurface of integrated circuit 204; that is, there are no voids, gaps,breaks or spaces in dielectric coating 102.

[0080] C. Conductive Coating

[0081] As noted, conductive coating 104 is the outer coating ofconformal EMI shield 100, providing the requisite EMI shielding for thecoated regions of the printed circuit board. As such, conductive coating104 is applied to the surface of dielectric coating 102 which has beenapplied previously to selected regions of the printed circuit board. Dueto the complete coverage provided by dielectric coating 102, conductivecoating 104 does not contact any portion of the printed circuit boardregion that has been coated previously by dielectric coating 102. Aswill be described in detail below, conductive coating 104 will alsolikely coat and adhere to predetermined portions of printed circuitboard, particularly ground pads, strips and moats (collectively, groundlands) on the printed wiring board.

[0082] In one preferred embodiment of conformal EMI shield 100,conductive coating 104 is TARA EMI-RFI shielding, Formula MQW-L85manufactured by The Egyptian Lacquer Manufacturing Company, Franklin,Tenn., USA. MQW-L85 is described in U.S. Pat. Nos. 5,696,196 and5,968,600 both of which are hereby incorporated by reference herein intheir entirety. MQW-L85 is designed for coating product enclosures orhousings such as those used in cellular phones.

[0083] Generally, conductive coating 104 has the capability to adhere tothe surface of dielectric coating 102 and, in certain embodiments, toground lands in the printed wiring board 202. Conductive coating 104 mayalso be required to adhere to other predetermined elements on theprinted circuit board in some applications. For example, in hybridshielding arrangements in which conformal EMI shield 100 is used inconjunction with a conventional metallic cage, conformal EMI shield 100may also be required to adhere to a surface of such a metallic cage.

[0084] As with dielectric coating 102, the material properties ofconductive coating 104 include primarily a combination of viscosity andadhesion. The combination of properties should be sufficient to enableconductive coating 104 to be applied via atomization spray techniquesand, once applied, to adhere to the surface of dielectric coating 102and, possibly, the other noted elements, in the immediate vicinity ofwhere it was applied. Specifically and in one embodiment, the viscosityof conductive coating 104 can range from 10-40″ Zahn cup #3 (fill body).In another embodiment, conductive coating 104 has a viscosity of 15-30″Zahn cup #3(fill body). MQW-L85 has a viscosity in the range ofapproximately 15-20″ Zahn cup #3(fill body) in other embodiments.

[0085] Conductive coating 104 has an adhesion suitable to enableconductive coating 104 to adhere to the noted materials, in particular,dielectric coating 102. As the viscosity of conductive coating 104decreases, the adhesiveness may need to increase to provide such acoating. In general, however, conductive coating 104 preferably has anadhesion that satisfies the ASTM 5B rating. To supplement the adhesionof conductive coating 104 to dielectric coating 102, it is preferredthat dielectric coating 102 and conductive coating 104 have a similarcomposite resin structures that facilitate bonding between the twocoatings. Such a bonding will be maintained over the life of the printedcircuit board due to the two coatings having similar coefficients ofthermal expansion. This reduces the shearing stresses between the twocoatings as the printed circuit board and, hence, conformal EMI shield100, heats and cools during the operational life of the printed circuitboard. For this reason, when dielectric coating 102 is the notedCQW-L200DF dielectric coating, it is preferred that conductive coating104 is the noted MQW-L85 due to the similarity of the composite resinstructures.

[0086] In one embodiment, conductive coating 104 is an aqueous coatingcomposition with particles of conductive metal suspended therein. Suchconductive metals can be, for example, copper, silver, nickel, gold orany combination thereof. The ohmic resistance of conductive coating 104is between 0.05 and 0.2 ohms per square at a film thickness ofapproximately 1.0 mil. The thickness of conductive coating 104 should besufficient to prevent the passage of the electromagnetic radiationgenerated by the coated printed circuit board 304. In one embodiment,the MQW-L85 conductive coating 104 is approximately 1.1±0.2 mils; thatis, a thickness in the range of 0.9 to 1.3 mils provides significantshielding effectiveness. However, it should be understood that inalternative embodiments, conducive coating 104 has a thickness thatdepends on its ohmic resistance and desired shielding effectiveness atthe anticipated electromagnetic frequencies to be shielded.

[0087] As with the preferred dielectric coating 102 noted above, MQW-L85is preferably applied at room temperature, between 70-80 degreesFahrenheit, although an application environment of 60-100 degreesFahrenheit is suitable. Preferably, multiple cross-coats are applied forone or more layers of conductive coating 104. After application, theMQW-L85 conductive coating 104 is cured at approximately 140-160 degreesFahrenheit for approximately 30 minutes. It should be understood thatlower temperatures can be used, depending on the temperature toleranceof the printed circuit board. The curing time may need to be alteredaccordingly. However, it is preferred that conductive coating 104 iscured at the noted temperatures because the elevated temperaturefacilitate the alignment of the metallic flakes. When the metallicflakes orient themselves in this way, the conductivity of the conductivecoating 104 is maximized.

[0088] A secondary effect of conductive coating 104 is that it isthermally conductive. The heat generated by coated printed circuit boardregions are transferred through dielectric coating 102 to conductivecoating 104 which conducts through the surface of the board. The heatcan then travel off the printed circuit board, primarily by dissipatingthrough convection or through conduction to a heat sink.

[0089] As with dielectric coating 102, conductive coating 104 can beapplied to the sharp edges and cavities of printed circuit board 304.This is illustrated in FIG. 2C in which conductive coating 104 coversdielectric coating 102 on integrated circuit 204. Conductive coating 104coats the side of integrated circuit body 206 behind leads 208, as wellas substantially all of the surface of leads 208 themselves. In thosecircumstances in which the gap between neighboring leads 208 is reduceddue to the presence of dielectric coating 102, conductive coating 104may bridge the gap as shown in FIG. 2C

[0090] 3. A Printed Circuit Board With A Conformal EMI Shield

[0091] A. General

[0092]FIG. 3 is a cross-sectional view of a portion of a printed circuitboard 304 with conformal EMI shield 100 of the present invention appliedthereto to cover the exposed surfaces of selected portions of printedcircuit board 304. Printed circuit board 304 of the present inventioncomprises, generally, printed wiring board 202 with components 302mounted thereon, with both shielded at least in part, and preferablycompletely, with conformal EMI shield 100 of the present invention. Inthe embodiment illustrated in FIG. 3, conventional metal cages 316A and316B are utilized to shield connector wires 320A and 320B of I/Oconnector 318. Conformal EMI shield 100 is applied to desired regions orportions of printed circuit board 304. Such regions or portions includeregions of printed wiring board 202 as well as all or part of certaincomponents 302.

[0093] Printed circuit board 304 includes a memory card 306 mounted onprinted wiring board 202. Memory card 306 is shielded by a conventionalmetal cage 316B. Printed circuit board 304 also includes integratedcircuit 204 introduced above with reference to FIGS. 2A-2C, a resistor310 and a power feed-through connector 308. Power feed-through connector310 carries low frequency signals and, therefore, does not need to beshielded. In contrast, another type of connector mounted on printedwiring board 202 is shielded connector 312. Connector 312 receives, forexample, high speed data signals. Shielded connector 312 has an EMIshield (described in detail below) whereas power feed-through connector308 does not.

[0094] A metal cage 316A shields I/O cables or leads 320A and 320B ofI/O connector 318. I/O connector 318 may be, for example, an RS232connector, among others. Metal cage 316A includes a surface-mountedfeed-through capacitor 314 for preventing signals from being conductedout of metal cage 316A on the low frequency signal traces to which it isconnected. Capacitor 314 has a lead in the form of solder spots and isconnected to a ground connection.

[0095] B. Printed Circuit Board Coverage

[0096] In accordance with one preferred embodiment, coated printedcircuit board 304 is completely shielded with conformal EMI shield 100.That is, conformal EMI shield 100 is a continuous coating covering allsurfaces of printed circuit board 304. However, conformal EMI shield 100need not cover the entire printed circuit board 304. For example, in oneembodiment, there may be regions of printed circuit board for which EMIprotection is unnecessary. In other circumstances, such as that shown inFIG. 3, other shielding mechanisms can be implemented on printed circuitboard 304 in combination with conformal EMI shield 100 to provide therequisite EMI shielding.

[0097] In FIG. 3 metal cages 316 are used to shield I/O connector 318leads 320 and memory card 306. In addition, ancillary parts of a productwhich generate minimal or no electromagnetic radiation do not warrantprotective measures to be employed to limit such emissions. Such devicesinclude, for example, interconnecting cables, power supplies, diskdrives, etc. These and other, similar devices do not need to be coatedwith the conformal EMI shield of the present invention. As a result,access to such components and subassemblies can be made simpler. Thus, aprinted circuit board 304 of the present invention is one that is atleast partially coated with one embodiment of conformal EMI shield 100.In FIG. 3, this conformal EMI shield 100 covers a portion of top surface322 of printed circuit board 304 in which components 302 are mounted, aswell as a bottom surface 326 of printed circuit board 304.

[0098] As one of ordinary skill in the art would find apparent,different techniques can be implemented to apply conformal EMI shield100 to specific regions of printed circuit board 304. For example, inone embodiment, conformal EMI shield 100 is selectively applied to thedesired portions of a printed wiring board or components mounted thereonusing highly directional air spraying techniques. Alternatively, printedcircuit board 304 is masked before application of the dielectric coating102 to avoid application to those regions of printed circuit board 304that are not to be shielded.

[0099] C. Grounding of Conformal EMI Shield

[0100] Conductive coating 104 is preferably grounded at variouslocations on printed circuit board 304. In the following embodiments,conformal EMI shield 100 is connected electrically to a ground plane inprinted wiring board 202. Two embodiments of making such a groundconnection are illustrated in FIGS. 4 and 5. FIG. 4 is a cross-sectionalview of a ground moat surrounding shielded connector 312 illustrated inFIG. 3. FIG. 5 is a cross-sectional view of a ground pad mounted onprinted wiring board 202.

[0101] Referring now to FIG. 4, conformal EMI shield 100 is preferablygrounded through a ground moat at locations where wires, leads, cables,etc., carrying high frequency signals are connected to printed wiringboard 202. Conformal EMI shield 100 effectively provides a conductiveloop around the signal wire connected to shielded connector 312. Acurrent can be induced in that portion of conductive coating 104surrounding shielded connector 312 due to the transmission of highfrequency signals through the connector. To prevent such a current fromtraveling to other portions of printed circuit board 304 or to emanateoff of the surface of conductive coating 104, ground moat 402 isprovided in printed circuit board 304 surrounding signal connector 312.To insure complete shielding, ground moat 402 preferably surroundscompletely shielded connector 312. One or more vias 406 connect groundmoat 402 to ground plane 404. In the embodiment shown in FIG. 4, thevias 406 are blind vias since they do not pass through to the other sideof printed wiring board 202. Preferably, there are a number of vias 406distributed around ground moat 402 to reduce the distance of theconductive path to ground plane 404. Any signals generated in conductivecoating 104 are immediately shunted to ground plane 404 through groundmoat 402 and a via 406.

[0102] Note that dielectric coating 102 is applied to printed wiringboard so as to not cover the surface of ground moat 402 and shieldedconnector 312. In one embodiment, this is achieved by masking groundmoat 402 and shielded connector 312 prior to applying dielectric coating102. Conductive coating 104 is applied so as to coat dielectric coating102 as well as ground moat 402. This is achieved by removing the maskfrom ground moat 402 and masking shielded connector 312 prior toapplying conductive coating 104. Importantly, either ground moat 402and/or conductive coating 104 are electrically connected to shield 408of shielded connector 312. Thus, any interference generated along thelength of the signal lead, connector or conductive coating 104 isimmediately shunted to ground.

[0103] Thus, a ground moat 402 mounted on printed wiring board 202completely around shielded connector 320 and connected electrically to ashield 408 of connector 312 and a ground plane 404 eliminates the EMIthat can be transmitted by conductive coating 104 in the vicinity ofshielded connector 312.

[0104]FIG. 5 is a cross-sectional view of a ground pad 502 mounted onprinted wiring board 202. In one embodiment, conformal EMI shield 100 isgrounded periodically through such ground pads 502 across all regions ofconformal EMI shield 100. In certain applications, the performance ofconformal EMI shield 100 is improved when it is grounded periodically.In one embodiment, this is achieved by providing one or more ground pads502 across the shielded regions of printed circuit board 304. One suchground pad 502 is illustrated in FIGS. 5, although there are many otherembodiments which can be implemented.

[0105] Ground pad 502 is a surface mounted conductive pad connected toground plane 404 through blind via 406. As with the embodimentillustrated in FIG. 4, dielectric coating 102 is applied to printedwiring board 202 so as to coat the surface of printed wiring board 202and not ground pad 502. Conductive coating 104 is applied so as to coatdielectric coating 102 and ground pad 502. This connects electricallyconductive coating 104 to ground plane 404.

[0106] In an alternative embodiment, vias 406 transect the entireprinted wiring board 202; that is, they extend from ground plane 404 toboth surfaces of printed wiring board 202. Accordingly, one embodimentof printed circuit board 304 is preferably arranged to take advantage ofsuch ground vias. For example, shielded connectors 312 and correspondingground moats 402 can be mounted on opposing sides of printed wiringboard 202. Alternatively, ground pads 502 or a combination of groundmoats 402, ground pads 502, ground strips or other combinations ofground lands can be disposed on opposing sides of printed wiring board202.

[0107] It should be understood that the location, quantity anddistribution of ground lands in general, and ground moats 402 and groundpads 502 specifically, can vary significantly depending on a number ofwell-known factors and features of printed circuit board 304. Forexample, the quantity of signal leads that come onto or off of printedwiring board 202, the frequency of the signals traveling on the signalleads. In addition, the resulting electromagnetic fields that aregenerated by the signals, which is based on the type of lead andconnector as well as the signal characteristics will also determine thegrounding scheme implemented. Referring to FIG. 3, for example, groundmoat 402 may be located at various locations on printed circuit board304, depending on the type of signals and components implemented. Forexample, ground moat 402 can be mounted on bottom surface 326 of printedwiring board 202 around the location at which I/O leads 320 enterprinted wiring board 202.

[0108] D. Electrically Connecting EMI Shielded Regions As noted,conformal EMI shield 100 can be applied to predetermined regions orportions of printed circuit board 304. Referring to the exemplaryembodiment illustrated in FIG. 3, conformal EMI shield 100 coats topsurface 322 of printed circuit board 304. This coating is physicallycontiguous, surrounding such elements as metal cages 316, shieldedconnector 312 and power feed-through connector 308. Similarly, in theembodiment disclosed in FIG. 3, conformal EMI shield 100 also coatsentirely bottom surface 326 of printed wiring board 202.

[0109] A potential can develop between the region(s) of conformal EMIshield 100 that coat top surface 322 and the region(s) of conformal EMIshield 100 that coat bottom surface 326. Should such a potentialdevelop, the two regions of conformal EMI shield 100 can effectivelyform an RF antenna and, therefore, be a source of EMI. To prevent thisfrom occurring, the top and bottom surface regions of conformal EMIshield 100 are preferably connected electrically to each other, directlyor through a common ground. Thus, conformal EMI shield 100 is anelectrically continuous coating that may or may not be physicallycontiguous over the surfaces of printed circuit board 304.

[0110] In the embodiment illustrated in FIG. 6A, the two regions ofconformal EMI shield 100 that coat the top and bottom surfaces ofprinted circuit board 202 are connected to each other through anotherregion of conformal EMI shield 100 applied to edge surfaces 324 ofprinted wiring board 202. In other words, the three regions (top, edgeand bottom coatings) can be considered a single region and printedcircuit board 304 is coated continuously on the top, edges and bottomsurfaces with conformal EMI shield 100. Thus, conformal EMI shield 100is, in this embodiment, physically contiguous and electricallycontinuous.

[0111] Should it be impracticable or otherwise undesirable to applyconformal EMI shield 100 to edges 324 of board 202, then alternativearrangements can be implemented to provide electrical continuity betweenall regions of conformal EMI shield 100. For example, in an alternativeembodiment shown in FIG. 6B, printed wiring board 202 can be made withplated edges. Edges 324 of board 202 are preferably plated with the samematerial as the material utilized in ground plane 404, such as copper.The top and bottom regions of conformal EMI shield 100 are connected toedge plating 604 on each side of board 202. As shown in FIG. 6B, edgeplating 604 wraps around printed wiring board 202 to form ground strips601 which cover some distance or area on top and bottom surfaces 322,326 thereof. As used herein, a ground strip 601 is an elongate groundpad.

[0112] Ground strips 601, edge plating 604 and ground plane 404 areconnected physically and electrically. Dielectric coating 102 is appliedto printed wiring board 202 so as to coat top and bottom surfaces 322,326 of printed wiring board 202 and not ground strips 601. Conductivecoating 104 is applied so as to coat dielectric coating 102 and at leasta portion of ground strips 601, as shown in FIG. 6B. This provides anelectrical connection between conductive coating 104 on the top andbottom surfaces 322, 326 to each other as well as to ground. Thus, inthis alternative embodiment, conformal EMI shield 100 includesphysically separate regions that are connected electrically through edgeplating 604.

[0113]FIG. 6C is a cross-sectional view of an alternative approach toachieving electrical continuity between regions of conformal EMI shield100. In this alternative embodiment, printed wiring board 202 ismanufactured with rows of ground vias 606 and one or more ground strips601 around its periphery. As noted, a ground strip 601 is an elongateground pad. On each side 322, 326 of printed wiring board 202, the vias606 are connected electrically to ground strips 601. As in theembodiment illustrated in FIG. 6B, dielectric coating 102 is applied toprinted wiring board 202 so as to coat top and bottom surfaces 322, 326of printed wiring board 202 while not coating ground strips 601.Conductive coating 104 is applied so as to coat dielectric coating 102and at least a portion of ground strips 601. This connects electricallyconductive coating 104 on the top and bottom surfaces 322, 326 to eachother as well as to ground.

[0114]FIGS. 6D and 6E are cross-sectional views of an edge region ofprinted wiring board 202 showing different embodiments for connectingelectrically regions of conformal EMI shield 100 that coat the top andbottom surfaces 322, 326 of printed wiring board 202 using spring clips.Specifically, in FIG. 6D ground strips 601 are mounted on top and bottomsurfaces 322, 326 of printed wiring board 202 proximate to edge surfaces324. One or more spring clips 602 are secured around edge 324 of printedwiring board 202 so as to contact ground strips 601 secured to opposingsides of printed wiring board 202. Sprint clip 602 is formed from anelectrically conductive material, and is preferably a unitary devicethat can be installed manually. As with the other embodiments,dielectric coating 102 is applied to printed wiring board 202 so as tocoat top and bottom surfaces 322, 326 of printed wiring board 202 whilenot coating ground strips 601. Conductive coating 104 is applied so asto coat dielectric coating 102 and at least a portion of ground strips601. This connects electrically conductive coating 104 on the top andbottom surfaces 322, 326 to each other through spring clip 602. Itshould become apparent that each ground strips 601 has a size or lengthsufficient to enable spring clip 602 to attach securely thereto, withoutrisk of inadvertent detachment.

[0115] In the embodiment illustrated in FIG. 6E, conformal EMI shield100 coats the entire top and bottom surfaces 322, 326 in the vicinityproximate to edge surface 324. In such embodiments, ground strips 601shown in FIG. 6D are eliminated, with spring clip 602 contactingdirectly conductive coating 104. As one of ordinary skill in the artwould find apparent, other configurations may be implemented to connectelectrically regions of conformal EMI shield 100 coating top and bottomsurfaces 322, 326 of printed circuit board 304.

[0116] E. Design of Printed Wiring Board to Accommodate EMI Shield

[0117] Aspects of the present invention include a printed wiring board202 constructed and arranged to operate with conformal EMI shield 100,as well as a printed circuit board 304 incorporating such a printedwiring board 202 and conformal EMI shield 100.

[0118] Printed wiring board 202 typically includes multiple layers eachof which includes an insulator, commonly an epoxy glass, with signaltraces and a ground plane formed on opposing surfaces thereon.Typically, traces internal to printed wiring board 202 are locatedbetween two ground planes with an intervening layer of insulatingmaterial. Signal traces that travel long the surface of the printedwiring board are positioned between a ground plane below, with anintervening layer of insulating material, and air above.

[0119] The characteristic impedance of the signal traces is a functionof the width and thickness of the trace, the distance between the traceand surrounding ground plane(s), and the dielectric properties of theintervening insulating material. The characteristic impedance in turneffects the electrical properties of the traces such as the velocity ofpropagation.

[0120] The greatest contributor to the characteristic impedance of asignal trace is the parasitic capacitance established between the signaltrace and its neighboring traces. Since internal traces have a groundplane located above and below it while a surface trace has a singleground plane located below it, the parasitic capacitance of the internaltrace is approximately twice that of surface traces, with a concomitantreduction in characteristic impedance.

[0121] This is not the case for printed circuit boards of the presentinvention. Coating a printed wiring board 202 with conformal EMI shield100 will increase significantly the parasitic capacitance of the surfacetraces, decreasing the characteristic impedance of the surface traces.The change in the characteristic impedance is, as noted, a function ofthe cross-section of the surface trace, the distance between the surfacetrace and conductive coating 104 and the dielectric properties ofdielectric coating 102.

[0122] Thus, in accordance with aspects of the present invention,printed wiring board 202 and conformal EMI shield 100 are configured tocontrol electrical characteristics of surface traces by adjusting suchfeatures. For example, the width and thickness of the surface traces aswell as the dielectric constant and thickness of dielectric coating 104can be adjusted to achieve desired electrical characteristics such ascharacteristic impedance. In an alternative embodiment, printed circuitboard 102 can be configured with no traces on the outer board layers.

[0123] In addition, a printed wiring board 202 of the present inventionincludes ground moats 402 mounted around connectors that may carry highfrequency signals, as described above, and, preferably, ground landsmounted periodically throughout printed wiring board 202.

[0124] 4. Individual Components Coated With Conformal EMI Shield

[0125] Repair of printed circuit boards 304 coated with conformal EMIshield 100 is likely to be difficult and expensive. The ideal solutionwould be to coat mainly the inexpensive parts of printed circuit board304, such that it would be economical to merely discard failed ordefective boards, salvaging the expensive processors, etc. for reuse.However, such components would lack the appropriate shielding. Aspectsof the present invention provide a technique for coating such componentswith conformal EMI shield 100 while enabling the components orsubassemblies to be removable for repair, replacement or salvage.

[0126]FIG. 7 is a cross-sectional view of an exemplary embodiment of aremovable component, memory card 306, coated with conformal EMI shield100. In this exemplary embodiment, a workstation or desk top computerprovides the capability to be configured more or less memory as neededfor the computer's particular application. To accomplish this, a printedcircuit board with memory sockets to receive various combinations ofmemory cards is included in the device. Such memory cards can be pluggedinto the socket and shielded with conformal EMI shield 100 with theother components 302 on printed circuit board 304. Alternatively, such amemory card can be shielded with a conventional metal cage 316. As shownin FIG. 3, when such a conventional metal cage is utilized, the cage isconnected to conformal EMI shield 100 through, for example, gaskets orflanges that are bonded to conformal EMI shield 100.

[0127] In accordance with another aspect of the invention, memory card306 can be coated individually; that is, conformal EMI shield 100 can beapplied to memory card 306 prior to it being installed in printedcircuit board 304. Embodiments of such aspects of the invention includea mechanism to electrically connect conformal EMI shield 100 coatingmemory card 306 with conformal EMI shield 100 coating printed wiringboard 202. In the embodiments shown in FIG. 7, such an electricalconnection is achieved through the use of mating shielded connectors 702and 312. As shown, connector 312 is physically and electricallyconnected to conformal EMI shield 100 applied to printed wiring board202.

[0128] Most computers need to accommodate accessory cards from variousvendors that add special capabilities. Examples include cards thatprovide the interface to a particular LAN protocol, or a high-speed datainterface. In one embodiment, the devices have special features tointerface with conformal EMI shield 100. In a more preferred embodiment,the devices are individually coated with conformal EMI shield 100, asdescribed above. In further embodiments, a a local shielding enclosure316 such as a metal enclosure with appropriate removable covers forinstallation of the accessory cards can be used. The interface betweenthe shielding of the metal enclosure and the coated board would be asdescribed above in connection with hybrid shielding arrangements, suchas gaskets between the enclosure and ground strips 603 on printed wiringboard 202.

[0129] 5. A Low Profile Component Cover For Encasing Components

[0130] It may be required or desired to access certain components 302mounted on printed wiring board 202. For example, during the operationallife of printed circuit board 304, it may be desired to access acomponent 302 for troubleshooting, repair or replacement. Also, it maybe desired to salvage a component 302 at the end of the operational lifeof printed circuit board 304. Such components 302 may include, forexample, expensive or rare components.

[0131] As noted, conformal EMI shield 100 completely coats thosesurfaces to which it is applied. Removal from printed wiring board 202of a component 302 coated with conformal EMI shield 100 requires thatshield 100 be severed at those locations where the component isconnected or adjacent to printed wiring board 202. For example,referring to FIG. 2C, this may include the boundaries between printedwiring board 202 and integrated circuit body 206 and leads 208.

[0132] There are a number of currently available techniques that couldbe used to sever conformal EMI shield 100. One such conventionalapproach is to chemically etch or otherwise dissolve conformal EMIshield 100. Unfortunately, such treatments typically include the use ofchemicals that are sufficiently active not only to penetrate conformalEMI shield 100, but to also damage the coated components 302. Inaddition, the accuracy of the application is limited, making itdifficult to precisely apply the chemicals to remove specific areas ofconformal EMI shield 100. As a result, severing conformal EMI shield 100at component-board boundaries around, for example, component leads,would be inefficient.

[0133] Another conventional technique that could be used to severconformal EMI shield 100 is referred to as sandblasting or, moreparticularly, as bead blasting. However, such an approach also lacksprecision and risks damage to the coated component 302, particularlyfragile components. Furthermore, even if components 302 can besuccessfully removed from printed wiring board 202, all surfaces ofcomponent 302 including its body and leads, will be coated withconformal EMI shield 100, as noted above. This may interfere with theintended activities or future use of the component.

[0134] There are two options currently available to avoid such drawbacksof traditional approaches. One approach is to not coat fragile andexpensive components 302 with conformal EMI shield 100, in which casethe component would not be shielded. An alternative approach is tocontain the component 302 within a conventional metallic cage 316, inwhich case it will suffer from the drawbacks noted above. Aspects of thepresent invention described below overcome the above and other drawbacksof chemical etching and bead blasting while not preventing the use andattendant benefits of conformal EMI shield 100.

[0135] In one aspect of the invention, a pre-manufactured,non-electrically-conductive, low profile component cover is secured toprinted wiring board 202, forming a sealed compartment dimensioned toencase component 302. Conformal EMI shield 100 can then be applied tothe exterior surface of the component cover in the manner describedabove. Since the component cover has a low profile, the coveredcomponent 302 experiences the same benefits of conformal EMI shield 100as if covered directly with conformal EMI shield 100. Here, however,conformal EMI shield 100 will not interfere with future uses of thecovered component. At least a portion of the cover, along with conformalEMI shield 100 attached thereto, can be easily removed from printedcircuit board 304 to expose component 302. Component 302 is thereafteraccessible, and can be tested or removed from printed wiring board 202using conventional techniques. In sum, components enclosed in acomponent compartment of this aspect of the invention are accessiblewhile enjoying the many advantages of conformal EMI shield 100.

[0136]FIG. 8A is a cross-sectional view of one embodiment of a component302 disposed in a sealed component compartment 804A formed by placing anon-electrically-conductive, low profile component cover 802A overcomponent 302, and securing component cover 802A to printed wiring board202. Component cover 802A in FIG. 8A has a surface of rotation about avertical axis 828, defining, in this embodiment, a symmetricalhalf-sphere. In an alternative embodiment shown in FIGS. 8B-8D a morearbitrarily shaped component cover 802B is shown. There, component cover802B forms with printed wiring board 202 an arbitrarily-shaped componentcompartment 804B for a processor integrated circuit 850.

[0137] In the embodiment illustrated in FIG. 8A, nonconductive componentcover 802A is preferably a pre-manufactured cover with a dome 822configured to envelop a selected component 302, and a flange 812configured to be secured to printed wiring board 202. Dome 822 has aclosed top 806, an open bottom 810 remote from top 806, and walls 808extending between closed top 806 and open bottom 810, forming a recess818 suitable for receiving component 302. Flange 812 surrounds openbottom of dome 822 and has a generally planar bottom surface 814 to matewith printed wiring board 202. When attached to printed wiring board202, component cover 802A and printed wiring board 202 form componentcompartment 804A. Component cover 802A can be unitary, or dome 822 andflange 812 are separately manufactured pieces that are attached to eachother to firm an integral cover. Dome 822 and flange 812 can bedetachably or permanently connected using an appropriatenon-electrically-conductive adhesive.

[0138] Component cover 802A is sealed to printed wiring board 202.Preferably, the junction between component cover 802A and printed wiringboard 202 are sealed so as to prevent dielectric coating 102 frompenetrating component compartment 804A. Preferably, componentcompartment 804A is evacuated and sealed to remove moisture fromcompartment 804A and prevent corrosion of component 302. Any commonlyknown technique can be used to create a vacuum in compartment 804A. Forexample, the same technique as that commonly used to mount an integratedcircuit can on a printed wiring board can be used.

[0139] As noted, one important feature of component cover 802A is thatit not prevent access to covered component 302. In one embodiment,component cover 802A is sufficiently thin and formed from a materialthat can be cut manually. In an alternative embodiment illustrated inFIG. 8A, a line of severability 816 traverses component cover 802A atthe boundary between dome 822 and flange 812. Preferably, line ofseverability 816 is a line of weakening that facilitates the severing ofdome 822 from flange 812, leaving flange 812 secured to printed wiringboard 202. In one embodiment, line of severability 812 is a crease, foldline or other weakened form. FIG. 8E shows two embodiments of a creaseline 824. In FIG. 8E-1, crease 824A is v-shaped groove pointing towardsthe interior corner formed by flange 812 and wall 808. In FIG. 8E-2,crease 824B is directed laterally across wall 808. Such embodimentssubstantially reduce the thickness of component cover 802A at thatlocation, facilitating severing of the portion of the cover traversed bythe line of severability, here, dome 822. Such severing may be achievedby scoring conformal EMI shield 100 (not shown) and cover 802A. Incertain embodiments, the material, wall thickness and depth of crease824 may be sufficient to enable a technician to score conformal EMIshield 100 and sever dome 822 manually.

[0140] It should be understood that the location and type of line ofseverability 816 can be selected for a given application. For example,the noted embodiments of line of severability 816 do not provide anopening into compartment 804A. Such embodiments enable compartment 804Ato be evacuated, as noted above. However, should component 302 not besubject to corrosion or otherwise benefit from such an evacuation, thenline of severability 816 could be implemented as a line of perforationsor other embodiment which partially penetrates the walls 808 of dome822.

[0141] Returning to FIG. 8A, in an alternative embodiment, dome 822 ofcomponent cover 802A is pressure-rupturable. When walls 808 aresubjected to a manual force applied radially inward, dome 822 rupturesand is severed along line of severability 816. In such embodiments, theinterior surface 820 of dome 822 would not touch the component 302 asshown in FIG. 8A; rather, a space sufficient to enable the ruptured dome822 to separate from flange 812 would be provided. Thus, in such anembodiment, to expose component 302, conformal EMI shield 100 is cut atthe junction between dome 822 and flange 812. In those embodiments inwhich line of severability is a crease, the crease can guide the pointof a knife or other cutting instrument. Manual force is then applied towalls 808 adjacent to flange 812, severing dome 822 from flange 812.Dome 822 is thereafter removed to expose component 302.

[0142] Preferably, recess 818 is dimensioned to receive component 302with minimal space between the interior surface 820 of dome 822 andcomponent 302 when component cover 802A is secured to printed wiringboard 202. This, in conjunction with the relatively thin top 806, walls808 and flange 812, results in a component compartment 804A having aminimal profile. In other words, the volume of compartment 804A in notsubstantially greater than the volume defined by the surfaces ofcomponent 302.

[0143] An important feature of component cover 802A is that it have ashape suitable for receiving dielectric coating 102 and, ultimately,conductive coating 104, while providing this minimal profile. As such,the exterior surface 826 of component cover 802A is preferably withoutsharp edges, indentations, or other abrupt changes. Thus, dome 822 cantake on virtually any shape beyond the symmetrical half-sphere shapeillustrated in FIG. 8A. For example, dome 822 can be disk-shaped,elliptical, rectangular and the like.

[0144] In another embodiment illustrated in FIGS. 8B-8D, a componentcover 802B has a contoured, arbitrary shape. FIG. 8B is across-sectional view of component cover 802B dimensioned to cover aprocessor IC 850. FIG. 8C is a cross-sectional view of component cover802B with a dielectric coating 102 covering the exterior surfacethereof, while FIG. 8D is a same view showing a conductive coating 104covering dielectric coating 102 forming conformal EMI shield 100 of thepresent invention.

[0145] Referring to FIG. 8B, there is no distinctive boundary betweendome 822B and flange 812B due to the contoured shape. A line ofseverability (not shown) can be formed in component cover 802B at anylocation above where flange 812B is attached to printed wiring board202.

[0146] Referring to FIG. 8C, dielectric coating 102 is applied to thesurface of printed wiring board 202 and exterior surface 826B ofnonconductive conformal cover 802B. Similarly, as shown in FIG. 8D,conductive coating 104 is applied so as to cover entirely dielectriccoating 104 applied previously to cover 802B.

[0147] Component cover 802 is, as noted, pre-manufactured withdimensions suitable for covering completely a particular component 302.Component cover 802 can be formed, folded or molded using any well-knowntechnique suitable for the material used and intended application. Withregard to materials, component cover 802 can be manufactured using anycombination of non-conductive materials. For example, component cover802 can be formed of polyethylene terephthalate (PETE),polyphenylsulfone (PPS) or RTV silicone rubbers, and polymers andsynthetic rubbers such as TEFLON and VITON, among others. (TEFLON andVITON are registered trademarks of E. I. Du Pont de Nemours andCompany.)

[0148] In alternative embodiments, component cover 802 is configured toprovide access to component 302 without severing component cover 802.For example, in one alternative embodiment, component cover 802 isformed with an aperture at top 806 and includes in combination a cover,beveled insert or the like that can be removeably inserted into theaperture. To gain access to component 302, conformal EMI shield 100around the beveled insert is scored and the insert removed. Whencomponent cover 802 is to be subsequently shielded, the beveled insertis reintroduced into the aperture and conformal EMI shield 100 isreapplied to component cover 802.

[0149] Thus, the low profile, non-electrically conductive componentcovers 802 enable components 302 to be shielded with conformal EMIshield 100 located at a location immediately adjacent to the components,in the near or induction field. In addition, component cover 802 doesnot provide any EMI shielding function, enabling a myriad of materialsand manufacturing techniques to be used make such covers.

[0150]FIG. 11 is a flow chart of the primary operations performed inutilizing a component cover shown in FIGS. 8A-8E with conformal EMIshield of the present invention. At block 1102 the exterior dimensionsof the component is determined. This includes all features of thecomponent, including leads, heat sinks, etc. this information is used todetermine the shape and size of dome 822 of component cover 802.Similarly, to determine the appropriate dimensions of flange 812, thespace around the component is measured at block 1104. From thismeasurement, the size and shape of flange 812, including theconfiguration of bottom surface 814 are determined.

[0151] At block 1106 the component cover 802 is manufactured based onthe dimensions determined at blocks 1102 and 1104. Alternatively,component 302 and component cover 802 can be specified prior to themanufacturing of printed wiring board 202. In such embodiments, printedwiring board 202 is manufactured to accommodate flange 812 of componentcover 802.

[0152] The component compartment 804 is formed at block 1108. Here,component cover 802 is attached to printed wiring board 202 to formcomponent compartment 804 dimensioned to encase component 302. Componentcompartment 804 is preferably evacuated or filled with a suitable inertatmosphere and sealed to maintain the environment at least untildielectric coating 102 is applied to component cover 802.

[0153] Conformal EMI shield 100 is applied at blocks 1110 and 1112. Atblock 1110, dielectric coating 102 is applied to printed wiring board202 and the exterior surface of component cover 802. The manner in whichdielectric coating 102 is applied is described elsewhere herein. Asnoted, dielectric coating 102 can be applied in many layers each bondedwith its neighboring layers to form dielectric coating 102. At block1112, conductive coating 104 is applied to the surface of dielectriccoating 102. Each step 1110 and 1112 includes a number of subsidiarysteps to prepare the surface, cure the coating, etc. This is describedin greater detail above. Thus, upon the completion of the operationsnoted in block 1112, a conformal EMI shield 100 is applied to thecomponent 302 contained in the component compartment. Since thecomponent compartment is constructed and arranged to have a low profile,it defines a volume not substantially different that the volume definedby the surface of the covered component. As a result, conformal EMIshield 100 remains in the induction region immediately adjacent tocomponent 302.

[0154] 6. Filler Material For Use With Board-Level Containment ofElectromagnetic Emissions

[0155] There are small gaps or spaces between component leads,neighboring components and between components 302 and printed wiringboard 202 that are relatively small. These various spaces are referredto herein generally and collectively as “cavities.” Such cavities mayhave more than one opening to the surface of the printed wiring board.For example, the space between the leads of a component and thecomponent body and printed wiring board is considered to be a cavity.Such a cavity has an opening to the surface of the printed circuit boardbetween neighboring leads. Significantly, dielectric coating 102 has acombination of properties that enables it to penetrate or access suchcavities. Dielectric coating 102 attaches to the component and printedwiring board surfaces forming such cavities when applied via airatomizing techniques, as described above.

[0156] Although dielectric coating 102 sufficiently coats the componentand board surfaces defining cavities, such surfaces are the moredifficult surfaces to coat with conformal EMI shield 100. In one aspectof the invention, an alternative approach is taken. Anon-electrically-conductive high viscosity material is applied tospecific regions of printed circuit board 304 to further insure cavitieson the printed circuit board are insulated prior to application ofconductive coating 104. This aspect of the invention will be describedwith reference to FIGS. 9A-9D. FIG. 9A is a cross-sectional view of twocomponents 302 mounted on a printed wiring board 202. In this example,one cavity 900A is located below the bottom surface of raised component914A while two additional cavities are beneath the leads 906 ofcomponent 914B.

[0157] Those components or groups of components 914 that have or formsuch cavities 900 with each other and/or printed wiring board 202 arecovered at least partially with a viscous, non-electrically-conductivefiller material 902. Filler material 902 bridges across the opening(s)of each cavity 900 to cover, enclose, encapsulate and seal the cavity.Oftentimes, the cavities 900 are also at least partially infilled withfiller material 902. Referring to the exemplary application shown inFIG. 9A, for example, cavities 900A and 900B are infilled while cavity900C is not. Regardless of whether a cavity 900 is infilled, however, acoating of filler material 902 eliminates the requirement thatdielectric coating 102 penetrate cavities 900 to coat component andboard surfaces defining the cavity 900. In addition, filler material 902can also be applied to highly variable surfaces of printed circuit board304. Highly variable surfaces include surfaces having a surface tangentthat experiences substantial changes in value and/or abrupt changes insign over small regions.

[0158] Selective applications of filler material 902 converts theirregular and cratered printed circuit board surface to a contiguoussurface having gradual transitions due to the covering of cavities andthe smoothing of sharp and abrupt surfaces. In other words, a printedcircuit board 304 having filler material 902 applied thereto has asurface tangent that does not change abruptly and which does not havecavities. Dielectric coating 102, when applied to components coveredwith filler material 902 will coat completely such components due to thecontiguous, contoured surface provided by filler material surface 912.Thus, filler material 902 insures the successful insulation of printedcircuit board 304 prior to the application of conductive coating 104.

[0159] Although the viscosity can vary, filler material 902 ispreferably thixotropic, enabling it to be extruded into and overcavities 900 while covering the top, side and other surfaces ofcomponents 914. In one embodiment, filler material 902 is an epoxy suchas any epoxy from the family of Bisphenol-A epoxies mixed with an aminehardner. In one particular embodiment, filler material 902 is an EMCAST,CHIPSHIELD, 3400-2500 and 3600 series epoxies available from ElectronicMaterials, Inc., Breckenridge, Colo. A thermally cured epoxy ispreferred due to the inability to directly apply UV radiation to fillermaterial 902 that is disposed in cavities 900 due to shadows cast by thecomponents.

[0160] In another embodiment, a latex based non-electrically conductivecoating, such as HumiSeal TS300 epoxy, sold under the tradenameTEMPSEAL, available from HumiSeal, Woodside, N.Y. In contrast to theBisphenol-A epoxies noted above, HumiSeal TS300 can be removed fromprinted circuit board 304 by manually peeling it from the componentsurfaces. In another embodiment, the epoxy ABLEBOND 9349K available fromTra-Con, Inc. is utilized as filler material 902. This epoxy is a gray,two-part epoxy manufactured with glass bead spacers to control the bondline thickness.

[0161] It should be understood that due to variations in material,surface cavity configuration, application technique or a combinationthereof, filler material 902 may cure with one or more voids. Forexample, referring to FIG. 9A, filler material 902 did not bridgecompletely across neighboring leads 906 in certain locations, formingvoids 904A and 904B. FIG. 9B is a top view of void 904A. As shown,filler material 902 fills the cavity 900A between and below neighboringleads 906. Void 904A extends into the space between leads 906, exposinga portion 908 of lead 906A. If conductive coating 104 were to be appliedto filler surface 912, void 904A would result in a short circuit of theexposed component lead 908. Thus, although such a circumstance can beeliminated through controlled processes, dielectric coating 102 ispreferably applied to all surfaces of printed circuit board 304,including surface 912 of filler material 902. This insures that voids904, if any, are completely insulated from the subsequently appliedconductive coating 104. FIG. 9C is a cross-sectional view of thecomponents shown in FIG. 9A, with a dielectric coating 104 applied tofiller surface 912 of filler material 902 and the surface of printedwiring board 202. As shown in FIG. 9C, dielectric coating 104 coverscompletely surface 912 of filler material 902, including voids 904. Asused herein, such voids, when coated with dielectric coating 104, arereferred to as insulated voids 910. Application of conductive coating104, as shown in FIG. 9D, results in a conformal EMI shield 100 thatcompletely covers, while being electrically isolated from, printedcircuit board 304.

[0162] It should be understood that the method for applying fillermaterial 902 is a function of the selected material and specified by themanufacturer. Other operations may be included as well. For example, toavoid the formation of air pockets within or below the filler material902 adjacent to components, the surface to be coated is subjected tonegative pressure prior to the application of filler material 902. Thiseliminates the possibility of trapping air where it could corrodecomponent surfaces. It should also be understood that multiple fillermaterials 902 can be incorporated into an EMI protected printed circuitboard, for example, when different filler materials have differentcombinations of viscosity, thermal conduction and other properties eachsuitable for coating different components.

[0163] 7. Manufacturing of Printed Circuit Board With Conformal EMIShield

[0164]FIG. 10 is a flow chart of the primary operations performed inaccordance with one embodiment of the present invention to form aprinted circuit board with a conformal EMI shield coating at least aportion thereof. In the exemplary process 1000 that follows the printedcircuit board is completely covered by conformal EMI shield 100.

[0165] Printed circuit board 304 is manufactured in steps or blocks 1002and 1004. In block 1002 printed wiring board 202 is formed. Printedwiring board 202 may include surface traces designed to transfer signalswhile coated with conformal EMI shield 100. Further, printed wiringboard 202 is constructed with ground pads in predetermined locations tobe connected to conductive layer 104. Optionally, printed wiring board202 may also include a series of ground vias located along the peripheryof printed wiring board 202 to insure the electrical continuity ofconformal EMI shield 100. At block 1004 printed wiring board 202 ispopulated with components to form one or more circuits, the sum of whichis printed circuit board 304.

[0166] Printed circuit board 304 is then prepared for the application ofconformal EMI shield 100 at block 1006. For example, soldering residuesthat may interfere with the ability of dielectric coating 104 to adhereto printed circuit board 304 are preferably washed off printed circuitboard 304.

[0167] At block 1008 highly viscous filler material 902 is applied topredetermined components to fill and cover cavities thereof, as well ascavities between neighboring components and between components andprinted wiring board 202. Filler material 902 may cover or encapsulatethe component(s) or group of components. Filler material 902 can beapplied using any well known extrusion technique that will not damagethe covered components 302.

[0168] At block 1010 one or more component covers 802 are mounted onprinted wiring board 202 to cover certain, predetermined components. Asnoted such components include those that are fragile or expensive andfor which access need be provided without interference from conformalEMI shield 100.

[0169] Dielectric and conductive coatings 102, 104 are applied at blocks1012 and 1014, respectively. One embodiment of the selected materialsand associated application process, are described above. Both,dielectric coating 102 and conductive coating 104 are likely applied topredetermine regions of printed circuit board 304. This can be achievedby masking the printed circuit board 304 or by using a precision sprayapplication technique, selectively applying coatings 304 to the desiredregions of printed circuit board 304. For example, when masking is used,after dielectric coating 102 has cured, any masking unique to dielectriccoating 102 is removed. Printed circuit board 304 is re-masked asnecessary to prevent conductive coating 104 from shorting out connectorcontacts, etc. Thereafter, this masking is also removed from printedcircuit board 304.

[0170] 8. Closing

[0171] While various embodiments of the present invention have beendescribed above, it should be understood that they have been presentedby way of example only, and not limitation. For example, although airatomization spray techniques are commonly used, the present inventioncan be applied to printed circuit boards 304 using other gases such asnitrogen. As another example, it has been disclosed that conformal EMIshield 100 is preferably grounded, such as to ground plane 404 inprinted wiring board 404. It should be appreciated, however, thatconformal EMI shield 100 may need to be connected electrically to anyreference voltage of which ground is only one. Thus, the breadth andscope of the present invention should not be limited by any of theabove-described exemplary embodiments, but should be defined only inaccordance with the following claims and their equivalents.

What is claimed is:
 1. An electrically continuous conformal EMIprotective shield for coating a region of a printed circuit boardcomprising: a low viscosity, high adherence dielectric coatingconfigured to be applied directly to surfaces of one or more regions ofthe printed circuit board, said dielectric coating configured to providea layer of insulation that adheringly coats all surfaces of the printedcircuit board region; and a low viscosity conductive coating configuredto be applied at least to said dielectric coating to preventelectromagnetic emissions generated by the printed circuit board fromemanating beyond the conformal coating, wherein said EMI shield adheresdirectly to and conforms with the surface of the printed circuit boardregion.
 2. The conformal EMI shield of claim 1, wherein said dielectriccoating is comprised of a dielectric material that is thermallyconductive.
 3. The conformal EMI shield of claim 1, wherein saiddielectric coating has a combination of adhesion and viscosity thatenables said dielectric coating to be applied with atomization spaytechniques so as to access and adhere to all exposed surfaces of saidone or more regions of the printed circuit board.
 4. The conformal EMIshield of claim 3, wherein said dielectric coating has a combination ofviscosity and adhesion properties sufficient to enable said dielectriccoating to be applied via atomization spray techniques to adhere to thesurface in the immediate vicinity of where it was applied.
 5. Theconformal EMI shield of claim 3, wherein said dielectric coating iscomprised of a plurality of successively-applied layers of dielectricmaterial.
 6. The conformal EMI shield of claim 1, wherein saiddielectric coating is thixotropic.
 7. The conformal EMI shield of claim4, wherein said dielectric coating has a viscosity of at least 45″ #2Zahn Cup (full body).
 8. The conformal EMI shield of claim 7, whereinsaid dielectric coating has a viscosity in the range of 50-100″ #2 ZahnCup (full body).
 9. The conformal EMI shield of claim 1, wherein saiddielectric coating has a viscosity of 70-95″ #2 Zahn Cup (full body).10. The conformal EMI shield of claim 1, wherein said exposed surfacesinclude cavities and surfaces having tangents that are highly variable.11. The conformal EMI shield of claim 4, wherein said dielectric coatinghas an adhesion that enables it to pass the ASTM D-3359-83 Method A TapeTest using a 1″ (25 mm wide) semi-transparent pressure-sensitive tapewith and adhesion strength of 25-70 ounces per inch when tested inaccordance with ASTM Test Method D-3330.
 12. The conformal EMI shield ofclaim 4, wherein said dielectric coating has an adhesion that enables itto pass the ASTM D-3359-83 Method A Tape Test using a 1″ (25 mm wide)semi-transparent pressure-sensitive tape with and adhesion strength of30-50 ounces per inch when tested in accordance with ASTM Test MethodD-3330.
 13. The conformal EMI shield of claim 4, wherein said dielectricmaterial is comprised of Clear Water Reducible Barrier Coat, FormulaNumber CQW-L200DF, manufactured by The Egyptian Coating LacquerManufacturing Company, Franklin, Tenn., USA.
 14. The conformal EMIshield of claim 1, wherein said dielectric coating is approximately 2mils thick.
 15. The conformal EMI shield of claim 3, wherein saiddielectric coating is formed from multiple applications each forming alayer of dielectric coating approximately 1 mil thick.
 16. The conformalEMI shield of claim 1, wherein said conductive coating has a viscosityof approximately 10-40″ Zahn cup #3.
 17. The conformal EMI shield ofclaim 16, wherein said conductive coating has a viscosity ofapproximately 15-30″ Zahn cup #3.
 18. The conformal EMI shield of claim17, wherein said conductive coating has a viscosity of approximately15-20″ Zahn cup #3.
 19. The conformal EMI shield of claim 1, whereinsaid conductive coating has an adhesion that satisfies ASTM 5B rating.20. The conformal EMI shield of claim 19, wherein said conductivecoating adheres to said dielectric coating, ground pads and otherpredetermined elements mounted on said printed circuit board.
 21. Theconformal EMI shield of claim 20, wherein said other predeterminedelements of said printed circuit board comprise a metallic cage.
 22. Theconformal EMI shield of claim 1, wherein said dielectric coating andsaid conductive coating have a similar composite resin structures. 23.The conformal EMI shield of claim 1, wherein said conductive coating isan aqueous coating composition with particles of conductive metalsuspended therein.
 24. The conformal EMI shield of claim 1, wherein saidconductive coating has an ohmic resistance of between approximately 0.05and 0.2 ohms per square at a film thickness of approximately 1 mil. 25.The conformal EMI shield of claim 1, wherein said conductive coating hasa thickness of approximately 1.1÷0.2 mils
 26. The conformal EMI shieldof claim 1, wherein said conductive coating is TARA EMI-RFI shielding,Formula MQW-L85 manufactured by The Egyptian Lacquer ManufacturingCompany, Franklin, Tenn., USA.
 27. An electrically continuous conformalcoating for shielding a plurality of regions of a printed circuit boardfrom electromagnetic interference comprising: a conductive coatinghaving an ohmic resistance sufficient to prevent electromagnetic wavesfrom passing therethrough, wherein said conductive coating adheringlyand conformingly coats the surface of each printed circuit board region;and a dielectric coating interposed between said conductive coating andpredetermined portions of each printed circuit board region, whereinsaid dielectric coating completely insulates said predetermined portionsof said printed circuit board, wherein said conductive coatings of eachprinted circuit board region are electrically connected to conductivecoatings conforming coating and secured to the surfaces of other regionsof the printed circuit board.
 28. The conformal coating of claim 27,wherein said dielectric coating has a combination of adhesion andviscosity properties that enables said dielectric coating to adhere toall exposed surfaces of a printed circuit board to which it is applied.29. The conformal coating of claim 28, wherein said dielectric coatinghas a combination of viscosity and adhesion properties sufficient toenable said dielectric coating to be applied via atomization spraytechniques to adhere to the surface in the immediate vicinity of whereit was applied.
 30. The conformal coating of claim 29, wherein saiddielectric coating is comprised of a plurality of successively-appliedlayers of dielectric material
 31. The conformal coating of claim 27,wherein said dielectric coating is thixotropic.
 32. The conformalcoating of claim 28, wherein said exposed surfaces include cavities andsurfaces with a highly variable surface tangent.
 33. The conformalcoating of claim 27, wherein said dielectric coating has a thicknessthat is greater than approximately 1.4 mils.
 34. The conformal coatingof claim 28, wherein said conductive coating adheres to said dielectriccoating, ground pads and other predetermined elements mounted on saidprinted circuit board.
 35. The conformal coating of claim 27, whereinsaid dielectric coating and said conductive coating have similarcomposite resin structures.
 36. A printed circuit board comprising: aprinted wiring board; a plurality of components mounted on said printedwiring board; and an electrically continuous conformal coating forproviding an EMI-impervious shield conformingly and adheringly on theprinted circuit board, including a conductive coating that prevents theelectromagnetic waves from passing therethrough, said conductive coatingconformingly and adheringly coating the surface of one or more regionsof the printed circuit board, wherein said conductive coating of eachsaid region is electrically connected to each other, and a dielectriccoating interposed between said conductive coating and predeterminedportions of each said printed circuit board region, wherein saiddielectric coating completely insulates said predetermined portions ofsaid printed circuit board region.
 37. The printed circuit board ofclaim 36, wherein said conformal coating is applied to regions of theprinted circuit board defining regions of said conformal coating,wherein said regions of said conformal coating are connectedelectrically to each other.
 38. The printed circuit board of claim 37,wherein said regions of said conformal coating are physicallycontiguous.
 39. The printed circuit board of claim 37, wherein saidprinted circuit board comprises a plurality of grounding pads mounted insaid printed wiring board, wherein said conductive coating is connectedelectrically to said grounding pads, wherein aid ground pads areelectrically connected to a ground source of the printed wiring board.40. The printed circuit board of claim 39, wherein said printed wiringboard comprises a ground plane and wherein said grounding pads areconnected to said ground plane through a ground via.
 41. The printedcircuit board of claim 40, wherein said printed circuit board furthercomprises: a shielded connector mounted on said printed wiring board,said shielded connector connected to a shielded cable through whichsignals travel; wherein said ground pads comprise a ground moat mountedon printed wiring board substantially around said shielded connector andconnected electrically to a shield of said connector and said groundplane.
 42. The printed circuit board of claim 40, wherein said regionsof said conformal coating comprise: a first region coating at least aportion of a top surface of said printed wiring board; and a secondregion covering at least a portion of a bottom surface of said printedwiring board.
 43. The printed circuit board of claim 40, wherein saidground pads are mounted around a periphery of said printed wiring board,and wherein said first and second regions are connected electricallythrough said ground plane.
 44. The printed circuit board of claim 42,wherein said printed circuit board has edge plating connectedelectrically to said first and second regions of said conformal coating.45. The printed circuit board of claim 42, wherein said edge plating iselectrically connected to a ground plane of said printed wiring board.46. The printed circuit board of claim 42, wherein said electricalconnection between said first and second regions is provided by acombination of: a ground land mounted on said top surface and saidbottom surface of said printed wiring board proximate to the edge ofsaid printed wiring board; and a plurality of electrically conductivespring clips spaced around said printed wiring board so as to contactsaid ground lands on said top and bottom surfaces of said printed wiringboard.
 47. The printed circuit board of claim 42, wherein saidelectrical connection between said first and second regions is providedby a plurality of electrically conductive spring clips spaced aroundsaid printed wiring board so as to contact said conductive coating ofsaid first region and said conductive coating of said second region. 48.The printed circuit board of claim 42, wherein said printed wiring boardcomprises signal traces formed on the surface thereof, wherein one ormore of the following features are selected alone or in combination suchthat said surface signal traces have a desired characteristic impedance:width of said surface signal traces; thickness of said surface signaltraces; dielectric constant of said dielectric coating; and thickness ofsaid dielectric coating.
 49. The printed circuit board of claim 48,wherein said printed wiring board comprises signal traces formed onsurfaces of internal layers only of said printed wiring board.
 50. Theprinted circuit board of claim 40, wherein one or more components arecoated individually with a conformal EMI shield, wherein said componentshield is electrically connected to said conformal coating on saidprinted circuit board.
 51. The printed circuit board of claim 37,wherein said electrical connection is achieved through vias in theprinted wiring board.
 52. A method for coating a printed circuit boardcomprising: providing a printed circuit board; and coating said printedcircuit board with an electrically continuous conformal coating forproviding an EMI-impervious shield conformingly and adheringly on theprinted circuit board, including a conductive coating that preventselectromagnetic waves from passing therethrough, said conductive coatingconformingly and adheringly coating the surface of one or more regionsof the printed circuit board, wherein said conductive coating of eachsaid region is electrically connected to each other, and a dielectriccoating interposed between said conductive coating and predeterminedportions of each said printed circuit board region, wherein saiddielectric coating completely insulates said predetermined portions ofsaid printed circuit board region.